Superhydrophobicity, a property of a surface when it resists contact with water, has been a topic of intense research during the last decade due to its potential in a wide variety of applications, such as self-cleaning, liquid-solid drag reduction, and water repellency. Water repellency of superhydrophobic surfaces is often studied by droplet impingement experiments in which millimetric drops of water are impacted onto these surfaces and photographed. With appropriate surface design, droplets can be made to bounce off completely. However, the time taken to bounce off—hereafter referred to as the contact time—is critically important as mass, momentum, and/or energetic interactions take place between the droplet and the surface during the time of contact. For example, the energy required to device an airplane wing can be reduced if a water drop rebounds off the wing before it freezes.
Recent literature suggests there is a theoretical minimum contact time, tc. See M. Reyssat, D. Richard, C. Clanet, and D. Quere, Faraday Discuss., 2010, 146, pp. 19-33; and D. Quere, Nature Letters, 2002, 417, pp. 811. Specifically, models that estimate the effects of contact line pinning on contact time have found that the contact time scales
                              t          c                ≈                  2.2          ⁢                                    (                                                ρ                  ⁢                                                                          ⁢                                      R                    3                                                  γ                            )                                      1              2                                ⁢                      (                          1              +                              ϕ                4                                      )                                              (        1        )            where tc is the contact time of a drop, of radius R, density ρ, and surface tension γ, bouncing on a superhydrophobic surface with pinning fraction φ. Even if one were able to completely eliminate this surface pinning such that φ=0, there would still be a minimum contact time limited by the drop hydrodynamics.
New articles, devices, and methods are needed to decrease the contact time between a droplet and a surface for improved liquid repellency. Contact times less than the theoretical minimum have heretofore been believed to be impossible.